The present invention is directed to golf balls and, more particularly, golf balls comprised of multi-layer cores. The present invention is also directed to an improved method of making golf balls with multi-layer cores.
Generally, golf balls have been classified as wound balls or solid balls. Wound balls are generally constructed from a liquid or solid center surrounded by tensioned elastomeric material. Wound balls are generally thought of as performance golf balls and have a good resiliency, spin characteristics and feel when struck by a golf club. However, wound balls are generally difficult to manufacture when compared to solid golf balls.
Early solid golf balls were generally two piece balls, i.e., comprising a core and a cover. More recently developed solid balls are comprised of a core, a mantle layer and a cover, in order to improve the playing characteristics of the ball.
The prior art is comprised of a variety of golf balls that have been designed to provide particular playing characteristics. These characteristics are generally the initial velocity and spin of the golf ball, which can be optimized for various types of players. For instance, certain players prefer a ball that has a high spin rate in order to control and stop the golf ball. Other players prefer a ball that has a low spin rate and high resiliency to maximize distance. Generally, a golf ball having a hard core and a soft cover will have a high spin rate. Conversely, a golf ball having a hard cover and a soft core will have a low spin rate. Golf balls having a hard core and a hard cover generally have very high resiliency for distance, but are hard feeling and difficult to control around the greens. Various prior art references have been directed to adding a mantle layer or second cover layer to improve the playability of solid golf balls.
Other prior art golf balls have multiple core layers to provide desired playing characteristics. For example, U.S. Pat. No. 5,184,828 claims to provide a golf ball having two core layers configured to provide superior rebound characteristics and carry distance, while maintaining adequate spin rate. More particularly, the patent teaches an inner core and an outer layer and controlling the hardness distribution in the outer layer and in the inner core in such a way that the golf ball has a maximum hardness at the outer site of the inner core. The patent alleges that such a distribution of hardness in the core assembly allows high energy to accumulate at the interface region where the hardness is maximum. The patent further claims that the energy of the club face is efficiently delivered to the maximum hardness region and transferred toward the inner core, resulting in a high rebound coefficient. However, since golf balls having hard cores and soft covers provide the most spin, the distribution taught by this patent would result in a maximum core hardness at the interface when hit by a driver so that the ball has a relatively high driver spin rate and not very good distance. Since the ball in this patent has a softer outer core layer, the ball should have a lower spin rate for shorter shots such as an eight iron, where spin is more desirable. Thus, the ball taught by this patent appears to have many disadvantages.
In order to improve the playing characteristics of a solid golf ball, Kasco, Inc. just introduced a new ball called Rockets.RTM.. The Rockets.RTM. ball is comprised of a center, two layers and a cover. The center and the two layers are all comprised of polybutadiene rubbers.
In particular, tests on such balls have shown that golf balls are comprised of a center having a diameter of about 1.0 inch, a first layer having an average thickness of about 0.125 inch and a second layer having an average thickness of about 0.13 inch. The center has a Shore C hardness of about 59 at the center and 60 at the center midpoint. The first layer has a Shore C hardness of about 61, and the second layer has a Shore C hardness of about 73. The cover of the Rockets.RTM. golf balls are harder than 65 Shore D and the compression is about 88.
Based upon the parting lines at each layer, it appears that Kasco manufactures the Rockets.RTM. golf ball core by forming the center, compression molding the first layer around the center and compression molding the second layer onto the center and first layer. It appears that the cover is molded using a retractable pin injection mold. The problem with the Kasco method is that the golf balls thus formed have non-concentric cores. That is, the center of the ball is not concentric with the remainder of the ball and the layers do not have uniform thicknesses. More particularly, the first layer was measured to have a maximum thickness on one side of 0.139 inch and a minimum thickness on the opposing side of 0.106 inch. Thus, there was a variance of 0.033 inch in the thickness of the first layer. Similarly, the second layer was measured to have a maximum thickness of 0.155 on a first side and a minimum thickness of 0.113 inch on the opposing side. Therefore, there was a difference of 0.042 inch in the thickness of the second layer. Thus it is evident that there is a significant concentricity problem in these golf balls.